Modern wireless communication systems rely on packet based transmission, which permits real-time multiplexing of multiple connections from different mobile devices as a means to increase the system capacity. In general, speech traffic is encoded using a fixed-rate or variable-rate encoder. While various types of codecs can be considered, a four-state Enhanced Variable Rate Codec (EVRC), which generates full-rate, half-rate, quarter-rate and eight-rate frames is considered as an example. This codec has been standardized for voice support in the cdma2000 1x and High Rate Packet Data (HRPD) standards. Each VoIP packet (or a bundle of packets) can be mapped to a physical Orthogonal Frequency Division Multiple Access (OFDMA) resource entity/burst (e.g., such physical resource entities are called “bursts” in WiMAX) and may correspond to one or more Medium Access Control (MAC) Protocol Data Units (PDUs) (MAC PDUs).
Since VoIP packets are relatively small in size, a single MAC PDU may be preferred to transport the contents of one or more VoIP packets thus minimizing the header and cyclic redundancy check (CRC) overhead. This allows the Hybrid Automatic Request (HARM) mechanism to operate on a per-burst basis, since the Cyclic Redundancy Check (CRC) is embedded in the MAC PDU.
The MAC PDU overhead accounts for a number of bits that need to be transmitted along with a payload that contains the information bits. For example, in the Worldwide Interoperability for Microwave Access (WiMAX) standard, the MAC PDU overhead may be as large as ten (10) bytes, resulting from six (6) bytes MAC header overhead and four (4) bytes of CRC). Robust header compression (ROHC) may be employed to reduce the original forty (40) bytes of Real Transport Protocol/User Datagram Protocol/Internet Protocol (RTP/UDP/IP) overhead to four (4) bytes only.
For WiMAX, the total header/trailer overhead with and without packet bundling (bundling of only two packets illustrated) is represented in the table of FIG. 1. Thus, FIG. 1 illustrates that when VoIP packet bundling. is employed with only one (1) VoIP packet bundled (i.e., no bundling of multiple VoIP packets), the RTP/UDP/IP overhead comprises thirty-two bits (i.e., four (4) bytes), the MAC header comprises forty-eight bits (i.e., six (6) bytes), and the CRC comprises thirty-two bits (i.e., four (4) bytes) for a total of one-hundred-twelve bits (i.e., fourteen (14) bytes). If two VoIP packets are bundled into the VoIP bundle, RTP/UDP/IP overhead comprises two (2) times thirty-two bits=sixty-four bits, while the MAC header and the CRC remain the same size giving a total bundle size of one-hundred-forty-four bits (i.e., eighteen (18) bytes). Bundling of additional VoIP packets would similarly increase the size of the RTP/UDP/IP overhead and the VoIP bundle.
The table of FIG. 2 illustrates the VoIP packet sizes including overhead—with and without bundling—at the output of an example EVRC speech codec. Other EVRC speech codecs may be utilized. FIG. 2 illustrates for a particular EVRC vocoder rate, the corresponding probability of usage, 20 ms frame size, and the speech/payload plus header/trailer overhead with bundling of one (1) packet (i.e., no bundling) and the bundling to two (2) packets. For example, with no bundling, the example EVRC vocoder produces a two-hundred-eighty-three (283) bit VoIP packet, a one-hundred-ninety-two (192) bit VoIP packet, a one-hundred-fifty-two (152) bit VoIP packet, and a one-hundred-twenty-eight (128) bit VoIP packet at the full rate, half rate, quarter rate and eighth rate respectively. Other EVRC speech codecs may produce different size VoIP packets at the respective rates illustrated.